HYDROGRAPHY OF MAGELLAN AND FUEGIAN PASSAGES 23

SCI. MAR., 63 (Supl. 1): 23-34 SCIENTIA MARINA 1999

MAGELLAN-ANTARCTIC: ECOSYSTEMS THAT DRIFTED APART. W.E. ARNTZ and C. RÍOS (eds.)

Hydrographic features of Magellan and Fuegian inlandpassages and adjacent Subantarctic waters*

TARSICIO ANTEZANA

Departamento de Oceanografía, Universidad de Concepción, Concepción, Chile. E-mail: antezana@udec.cl

SUMMARY: Hydrographic features and bottom profiles along the main passages from Paso Ancho in the Straits of Mag-ellan to the Beagle Channel and those of adjacent oceanic waters are examined with regard to water origin and circulation.Sills and shallow entrances likely limited water exchange. Adjacent oceanic waters were warmest in the Atlantic and salti-est in the Pacific sectors. Waters within the inland passages were fresher and cooler than open shelf waters, showing adecreasing salinity gradient between the Beagle Channel and the Straits of Magellan and a subsurface wedge of warmestand saltiest oceanic water underneath a core of cold and brackish water in Brazo Noroeste. Subsurface water masses of theStraits of Magellan and Brazo Noroeste seem to be entrapped. Temperature and density distributions suggest that the inflowof salty and warm Pacific waters takes place through Bahía Cook. Westward and northward toward the Straits of Magellan,these waters may progressively mix with cooler and more brackish waters adjacent to Cordillera de Darwin. In this sector(Canal Brecknock-Canal Cockburn) stratification of the water column was weaker and became zero toward the Straits ofMagellan (Seno Magdalena / Paso Ancho). Distribution of water properties was consistent with bathymetry profiles and sug-gests the following subdivision of microbasins along the Magellan-Beagle passage: 1.- Paso Ancho-Seno Magdalena, 2.-Canal Magdalena-Canal Brecknock, 3.- Canal Ballenero-Brazo Noroeste, 4.- Beagle Channel .

Key words: Hydrography, bathymetry, straits, microbasins, estuaries, fjords, brackish, glacier waters.

RESUMEN: RASGOS HIDROGRÁFICOS DE LOS CANALES INTERIORES MAGALLÁNICOS Y FUEGUINOS Y DE LAS AGUAS SUBANTÁRTI-CAS ADYACENTES. – Las características hidrográficas y los perfiles batimétricos a lo largo del transecto Estrecho de Maga-llanes a Canal del Beagle se examinaron en relación al origen y circulación de las aguas. Las aguas oceánicas adyacentesfueron más cálidas en el Atlántico y más salinas en el Pacífico. El intercambio de agua parece limitado por bajos y pasossomeros. Las aguas interiores fueron más salobres y frías que las aguas oceánicas de la plataforma, presentando una dismi-nución gradual de la salinidad desde el Canal del Beagle al Estrecho de Magallanes y una cuña subsuperficial de agua máscaliente y salina por debajo de un núcleo de agua fría y salobre en el Brazo Noroeste. Las aguas subsuperficiales del Estre-cho de Magallanes y Brazo Noroeste parecen estar semi-aisladas. La distribución de salinidad y temperatura sugiere que elflujo de aguas calientes y salinas del Pacífico se produce en Bahía Cook. Hacia el oeste y el norte, estas aguas se mezclarí-an progresivamente con aguas más frías y salobres provenientes de la Cordillera de Darwin. En este sector (Canal Breck-nock-Canal Cockburn), la estratificación de la columna de agua fue más débil, y llegó a ser nula hacia el Estrecho de Maga-llanes (Seno Magdalena-Paso Ancho). La distribución de las propiedades hidrográficas y los perfiles batimétricos fueronconsistentes y sugieren la siguiente sub-división de microcuencas a lo largo del transecto Magallanes-Beagle: 1.- PasoAncho-Seno Magdalena, 2.- Canal Magdalena-Canal Brecknock, 3.- Canal Ballenero-Brazo Noroeste, 4.- Canal del Beagle.

Palabras clave: Hidrografía, batimetría, estrechos, microcuencas, fiordos, estuarios, aguas salobres, ventisqueros.

*Accepted January 4, 1999.

INTRODUCTION

The coast line of southern South America west ofthe Andes mountains was broken down by quater-nary glaciation to form a vast and long archipelagoextending to Cape Horn. The complex web of chan-nels, inlets and surrounding land masses form a sys-tem of basins whose hydrographic and ecologicalpeculiarities are poorly known. Fjords as a type ofestuary generally possess steep sides, rock bottoms(which may be thinly veneered with sediments), andunderwater sills (Kennish, 1986). They are there-fore, glacial troughs now partly occupied by the sea.

Located within the latitudinal range of the WestWind Drift and north of the Polar Front, this fjordsystem is influenced by Subantarctic adjacentwaters and also by precipitation, continental runoffand melting ice. According to Pickard (1971; 1973),Chilean fjords are atypical when compared toBritish Columbian and Alaskan fjords because ofless abundant freshwater runoff, the absence of sillsin most inlets and because of a very complex tem-perature structure of the water column.

Pickard (1973) and Panella et al. (1991) pointedout that the westward influence of Atlantic waters inthe Straits of Magellan extends through Paso Ancho,and the eastward influence of Pacific waters reachesIsla Carlos III, where a narrow constriction and ashallow sill exists (Antezana et al., 1992). Tidal cur-

rent driven exchange between oceanic and inlandwaters takes place to a different extent at both west-ern and eastern entrances (Panella et al., 1991).However, no description has been given of gradientsor frontal conditions offshore, since results of thosecruises did not cover adjacent oceanic waters.

The first objective of this work was to describethe characteristics of adjacent oceanic waters as areference base to infer the degree of isolation ofinland waters or their exchange between inland pas-sages (such as the Canal Magdalena / Canal Cock-burn / Canal Brecknock system and Canal Ballenero/ Canal Beagle system), and with oceanic waters.(Spanish names from Chilean navigational chartsare used in this study). These channels are connect-ed with the Pacific Ocean and water exchange isexpected to occur in peculiar ways which are notdescribed yet. Most recent studies based on hydro-graphic (Panella et al., 1991), biological (Saggiomoet al., 1994; Carrada et al., 1994; Cabrini and FondaUmani, 1991; Fonda Umani and Monti, 1991; Maz-zocchi and Ianora, 1991) or geological (Brambati etal., 1991) features, identified basins within theStraits of Magellan and proposed environmentalboundaries between them. Most of these boundariescoincided with a subdivision, based purely on bathy-metric and topographic features (Antezana et al.,1992), which anticipated significant effects of bothdepth and side contours on water exchange betweenbasins and with the open ocean.

24 T. ANTEZANA

FIG. 1. – Distribution of hydrographic stations along the Straits of Magellan - Beagle Channel passage on boardR.V. “Victor Hensen” in spring 1994.

The second objective of this study was to exam-ine bottom profiles and the distribution of waterproperties along these passages to identify basinswithin the system and to infer the extent of theirexchange with open ocean waters and betweenbasins. Particularly higher sampling resolution (sta-tions were more closely located) was achieved in amost unique area of Brazo Noroeste, where numer-ous glaciers reach the sea surface.

MATERIAL AND METHODS

The study area

The study area (Fig.1) extended along a transectdelimited by the normal navigation between PasoAncho (off Punta Arenas) and Canal Beagle (offPuerto Williams) through the following Magellanand Fuegian passages in the same N-S order: PasoAncho, Paso del Hambre, Seno Magdalena, CanalMagdalena, Canal Cockburn, Canal Brecknock,Canal Ballenero, Canal O’Brien, Brazo Noroesteand Canal Beagle. Bathymetry extended to connect-ing passages to the open ocean. The hydrographicdescription covered the area 1100 km offshore in theSubantarctic south eastern Pacific.

Paso Ancho is the widest part of the Straits ofMagellan situated midway between the Atlantic andPacific and extending toward the west into BahíaInútil and toward the southwest to Canal Whiteside/ Seno Almirantazgo. At the southernmost vertex ofthe V shaped Straits of Magellan, Paso del Hambreand Seno Magdalena narrow and extend southwardinto Canal Magdalena, and eastward to Canal Cock-burn. At this point the channel system opens to thePacific Ocean through Bahía Desolada. Eastwardinto Canal Brecknock, Canal Unión, Canal Ballen-ero and Canal O’Brien, the coast line is more brokento the north by small and narrow inlets and to thesouth by connections to the Pacific Ocean at BahíaFitzroy (at the junction of Canal Unión and CanalBallenero) and Bahía Cook (at the junction of CanalThompson and Brazo Sudoeste). Eastward intoBrazo Noroeste and Canal Beagle the passage is nar-rower and is broken and bordered by glaciers on thenorth side and a narrow passage on the west side ofIsla Navarino. On the east side of this Island the pas-sage opens widely toward the Cape Horn.

The main sources of continental fresh waterswithin this area seem to be associated withCordillera de Darwin, the southern extension of the

Andes mountains, located along the southwesternsector of Tierra del Fuego. Melting ice and precipi-tation waters likely entered the system through 1.-Seno Almirantazgo into Bahía Inútil and PasoAncho, 2.- Canal Gabriel and Seno Keats into CanalMagdalena and Seno Magdalena, and 3.- alonginlets and glaciers on the north side of BrazoNoroeste and Canal Beagle.

Data collection

This study is mainly based on data collected dur-ing the Magellan campaign of R.V. “Victor Hensen”leg 1 and 2 carried out in November 1994 (Arntzand Gorny, 1994; Antezana et al., 1996) during thedry season of the year. Additional data from thePiquero Expedition (Fig. 2) on board R. V. “ThomasWashington” during January-February 1969 (Anon,MS) and some published data from Atlantic watersadjacent to Tierra del Fuego (Reid, 1994; Sánchez etal., 1993) are also included.

On transit between Punta Arenas and PuertoWilliams (2-6 November), 14 stations were occu-pied at 5-10 miles intervals (Fig. 1) with greatergaps in Paso Ancho and Canal Ballenero and

HYDROGRAPHY OF MAGELLAN AND FUEGIAN PASSAGES 25

FIG. 2. – Schematic distribution of main currents and isobaths nearthe tip of South America and distribution of hydrographic stations

during the Piquero III expedition in summer 1969.

much smaller intervals in the glacier sectors ofBrazo Noroeste, particularly in front of glaciersGaribaldi and Italia. At every station echo-sound-ing measurements of bottom depth determined themaximum suitable depth for the STD cast whichwas 200-300 m in most cases. Tidal currents, windstress and uneven bottom topography preventedsampling closer than 20-30 m from the bottom. ASensor Data SD-202 STD was used at most sta-tions except for the stations in Paso Ancho wherea Sea Bird SBE19 was used. Previous calibrationsof these instruments gave reliable accuracy, rela-tive to the wide range of salinities found. The datawere recorded in analog format during STD

descent. Salinity sections are given in practicalsalinity units (psu), temperature in ºC and poten-tial density in kg/m3.

Two liter Niskin bottle casts at fixed depths of 0,2, 5, 10, 15, 50 m and in some stations 75 and 100m followed. Water samples were drawn for phyto-plankton identification and photo pigments andnutrient measurements. A bongo net cast to 50 mdepth followed and ended the station. Most stationstook place at night hours.

Bathymetry profiles were obtained from naviga-tional charts of the Chilean Hydrographic Institute,by plotting the best approximate depth value at onemile intervals along the axis of the passage.

26 T. ANTEZANA

FIG. 3. – Distribution of isolines during Piquero III expedition . a.- Temperature at 0 m, b.- Salinity at 0 m, c.- Density at 0 m ; d.- Salinity at 50 m.

RESULTS

Subantarctic waters surrounding South America

The main source of surface water approaching thesouthern cone of South America (Fig.2) comes fromthe West Wind Drift which deflects south to form theCape Horn Current, as seen from potential densityisolines (Fig.3c). Beyond Cape Horn, Subantarcticwaters as part of the circumpolar current system con-tinue eastward and northward, beyond the continen-tal shelf to form the Falkland (Mal-vinas) Current.Toward the coast, Subantarctic waters mix withchannel waters from rivers, glaciers, precipitationand continental runoff to produce a belt of brackishwaters surrounding the tip of South America (Fig.3b). This layer did not extend much deeper than 50m on the Pacific side (Fig. 3d). The surface salinityminimum on the Atlantic was associated with theoutflow of Straits of Magellan waters; it is very wide,extending across the entire Argentinian shelf. On thePacific side, the salinity minimum was very narrow,as Subantarctic oceanic waters are separated fromthe fjord system by a very narrow continental shelf.Furthermore, salinities were higher than on theAtlantic shelf. This difference could be accounted forby a greater contribution of deeper higher salinitywaters near the Pacific coast. The shallow salinityminimum layer associated with fjords was wider atthe middle and northern sectors of the fjord regionand narrowed southward to Cape Horn where it wasrestricted to a very thin layer. Therefore Subantarcticwaters reaching the Magellan channels have lessersalinity than those approaching the more southernFuegian channels.

Sea surface temperature distribution (Fig. 3a)showed similar tendencies as salinity (Figs. 3b, 3d)and potential density distributions (Fig. 3c). Associ-ated with the formation of the Cape Horn Current,isotherms form a wedge of warm water withdecreasing temperatures southward, with higher val-ues off the Magellan channels and lower values offthe Fuegian channels. Beyond Cape Horn, low tem-perature waters merging with the Antarctic Circum-polar Current flow north and were mostly excludedfrom the Argentinean shelf sector.

In conclusion, Subantarctic surface waters with-in the latitudinal range of the West Wind Drift haddifferent characteristics in the Pacific and theAtlantic sectors adjacent to the coast of South Amer-ica. On the extensive Patagonian shelf, surface tem-perature ranged between 8.4 and 9.4 ºC and salinity

between 32.7 and 33.7. Temperature of surfacewaters within 220 km off the Chilean Fuegian chan-nels ranged between 6.9 and 7.9 ºC and salinitybetween 33.8 and 34.1, while off the Magellan chan-nels temperature ranged between 7.5 and 9.5º C andsalinity between 33.6 and 33.8. Beyond the Argen-tinean shelf both temperatures and salinities ofPacific and Atlantic waters were more similar.

Depth profiles along the inland passages to theopen ocean

The main connections from these inland pas-sages to open Pacific waters as indicated in naviga-tional charts (Fig. 1) are: 1.- SW of Canal Cockburn(Bahía Sin Nombre), 2.- Canal Union (the SE exten-sion of Canal Brecknock), 3.- the confluence ofCanal Brecknock and Canal Ballenero (S of BahíaDesolada), 4.- half way along Canal Ballenero(through Bahía Fitz-Roy), 5.- Canal Thompson, (thesouthward extension of the confluence of CanalO’Brien / Brazo Noroeste), and Brazo Sudoeste(through Bahía Cook) and 6.- the easternmostentrance of Canal Beagle.

Without attempting a detailed description of thebottom profile, some features likely to affect watercirculation are pointed out (Fig. 4). Paso Anchobasin in the Straits of Magellan extends southwardbeyond Paso del Hambre to form Seno Magdalena,a shallower rise about halfway between the Pacificand the Atlantic. Southward, Canal Magdalena fol-lows a straight N-S direction and merges into CanalCockburn which runs westward to connect to thePacific.This section (Fig. 4a) is rather deep with amound of 200 m at the entrance of Canal Magdale-na and a net downward slope toward the open oceanwhich is separated by a 30 m sill. Canal Cockburnopens in a straight southerly direction but thisentrance is fringed by several small islands whichdivide the mouth of Canal Cockburn into severalshallow passages. The confluence with Canal Brec-knock is also connected to the ocean by the same sillof 30 m extending eastward.

The section along Canal Brecknock, Canal Bal-lenero, Canal O’Brien and Brazo Noroeste (Fig.4b)is relatively shallow and could be divided into twosectors by a rise of 30-50 m at the western end ofCanal O’Brien: 1.- Canal Brecknock / Canal Ballen-ero and 2.- Canal O’Brien / Brazo Noroeste.The for-mer sector with a zig-zag shape on a net WE direc-tion, is connected to the open Pacific waters in sev-eral locations on the southern side and to several

HYDROGRAPHY OF MAGELLAN AND FUEGIAN PASSAGES 27

inlets on the northern side. South of Bahía Desola-da, another area of fresh water influx on the S sideof Cordillera de Darwin, Canal Brecknock joinsCanal Ballenero. This wide opening is in fact alabyrinth formed by small islands with two mainpassages connecting inland waters from open Pacif-ic waters over a sill of 40 m. The former passagemerges into the western entrance of Canal O’Brienover a shallow sill (at Isla Redonda) and into CanalBallenero which is wide and deep. At their conflu-ence next to Isla Timbales, is Canal Thompson, a 20km long and 200-650 m deep passage which con-nects the Canal Ballenero / O’Brien / BrazoNoroeste system to the Pacific through Bahía Cook.This is the deepest connection to the Pacific of thestudy area. Brazo Noroeste is a deep basin reaching350-400 m and ending on the east in a narrow andshallow (80 m) constriction (at Isla del Diablo)which abruptly separates it from Canal Beagle in thePunta Yámana sector.

The section along Brazo Sudoeste / Canal Beagle(Fig.4c) could be divided into three sectors by apeak of 30 m half way along Brazo Sudoeste and byanother sill of 45 m at Punta Ushuaia. The depthprofile of the deep western sector extends into BahíaCook which is also connected to Canal Thompson.Bahía Cook is separated from the open Pacific by a40 m sill at Isla Hammond which is somewhat deep-er than other open ocean waterways of the studyregion. The eastern sector of Brazo Sudoeste has ashallow bottom which merges into Canal Beagle asa continuum. Eastward, Canal Beagle is progres-sively shallower particularly beyond PuertoWilliams (40-60 m) extending to the easternentrance for 75 km to open into the wide and shal-low continental shelf on Bahía Nassau next to CapeHorn.

In summary, since the connections of Pacificwaters to the inland passages are limited by sills orshallow plains of 30-40 m, continuity between pas-

28 T. ANTEZANA

FIG. 4. – Bottom profiles along three main Magellan and Fuegian passages. Depth in meters. a.- Pacific Ocean (Bahía Sin Nombre) / CanalCockburn / Canal Magdalena / Seno Magdalena / Paso del Hambre / Paso Ancho; b.- Pacific Ocean (Bahía Sin Nombre) / Canal Brec-knock / Canal Ballenero / Canal O’Brien / Brazo Noroeste; c.- Pacific Ocean (Bahía Cook) / Brazo Sudoeste / Canal Beagle / Pacific

Ocean (Bahía Nassau).

sages could be established in a single profile fromthe eastern entrance of Canal Beagle to Paso Anchoin the Straits of Magellan. Considering just depthprofiles and other topographic accidents such as lat-eral contractions along the passages, the followingsubdivisions of microbasins are proposed:

1.- Seno Magdalena extending to Paso Frowardand Paso Ancho, 2.- Canal Magdalena / Canal Cock-burn, 3.- Canal Brecknock to Canal Ballenero, 4.-Brazo Noroeste, 5.- Canal Beagle / Brazo Sudoeste.

Waters within main passages

Vertical distribution of temperature, salinity anddensity are depicted in the two main sections: SenoMagdalena / Canal Magdalena / Canal Cockburn /Canal Brecknock Passage (Fig. 5) and Canal Brec-knock / Canal Ballenero / Canal Noroeste / CanalBeagle Passage (Fig. 6). Data from Canal Breck-nock were plotted in both transects as they are atthe confluence of both passages. Data from coastalPacific waters from the Piquero expedition (in latespring) were included as coastal open ocean refer-ence stations.

Paso Ancho / Seno Magdalena / Canal Magdalena/ Canal Cockburn / Canal Brecknock Passage.

Distribution of isopycnals (Fig. 5a) showed anabrupt upward deflection toward Canal Brecknockand an abrupt downfall at the entrance of Canal Mag-dalena (Punta Anxious). The most homogeneouslydistributed densities of the entire study area occurredat Seno Magdalena (Isla Peak) where the 24.4 kg/m3

layer extended to 200 m. In and around Paso Anchothere was a slight water stratification associated witha lens of 24.4-24.2 kg/m3 in the upper 50 m. Verticaldistribution of densities near the Pacific entrance atthe confluence of Canal Cockburn and Canal Breck-nock presented a strong gradient.

The distribution of isohalines (Fig. 5b) mirroredthe density distribution with a similar abrupt front atthe Pacific entrance and a transition between homo-geneous conditions of Seno Magdalena to stratifiedconditions of the Canal Cockburn / Canal Brec-knock passage. The high salinity deep layer found inCanal Brecknock and Canal Cockburn graduallydisappeared to a completely mixed water column of31.1-31.3 salinity.

Temperature distribution (Fig. 5c) showed anabrupt difference between warm Pacific waters and

inland waters. The Paso Ancho / Paso del Hambre /Seno Magdalena sector showed the coldest watersand complete thermal homogeneity to 200 m (whichextended to 300 m according to the deepest data col-lected at these stations). The Canal Magadalena /Canal Cockburn / Canal Brecknock sector showedinstead a surface lens of warmer water. This was theonly sector of the entire study area where a shallowwarm water layer was found. A thermal front,shown by the upward deflection of the 6.4 ºCisotherm to the surface was observed in Seno Mag-

HYDROGRAPHY OF MAGELLAN AND FUEGIAN PASSAGES 29

FIG. 5. – Vertical distribution of water properties along the transectPaso Ancho / Canal Magdalena / Seno Magdalena / Canal Cockburn/ Canal Brecknock / Pacific Ocean in spring 1994. a.- Density; b.-

Salinity; c.- Temperature.

dalena, ca. 10 km north of the entrance of CanalMagdalena (Punta Anxious).

Canal Brecknock / Canal Ballenero / CanalO’Brien / Brazo Noroeste / Canal Beagle Passage

Distribution of densities (Fig. 6a) indicatedimportant gradients toward the Pacific and at theconfluence of Canal Noroeste and Canal Beaglenear Punta Yámana. A shallow lens of low potentialdensity (less than 24.5 kg/m3) occurred along theglacier sector of Brazo Noroeste. Nearby a deepwedge of high density impinged upward to about 40m in the eastern sector, but did not extend beyondthe confluence with Canal Beagle (and BrazoSudoeste) where the bathymetry profile indicated asill of 80 m. In the westernmost sector (at Isla Sid-ney), waters with similar densities were homoge-neously distributed in the water column to 90 m .

Vertical distribution of isohalines (Fig. 6b) alongthe same section mirrored the distribution of isopyc-nals. A lens of lowest salinities extended throughoutthe glacier sector of Brazo Noroeste to gradually dis-appear through Canal Ballenero and Canal Brec-knock. A subsurface layer of higher salinity watersoccurred here and was particularly obvious towardthe deepest and easternmost sector. This core diffusedwestward. It was barely present toward the eastern-most part of Brazo Noroeste (Ventisquero Italia) anddid not reach Punta Yámana on the Canal Beagle.

Vertical distribution of isotherms (Fig. 6c) alongthis section showed a subsurface warmer wedge ris-ing eastward with a core of >8 ºC in the glacier sec-tor of Brazo Noroeste. Overlying this warm layerthere was a cold upper layer (10-40 m of water lessthan 6ºC), which was thinner on the glacier sectorsand thickened westward. Both cold upper and warmdeep layers were separated by strong thermoclineswhich deepened in the E-W direction.

In summary, vertical sections of density, temper-ature and salinity along the Straits of Magellan toCanal Beagle suggest the subdivision of the follow-ing microbasins:

1.- Seno Magdalena / Paso del Hambre / PasoAncho, 2.- Canal Magdalena / Canal Cockburn /Canal Brecknock, 3.- Canal Ballenero / CanalO’Brien / Brazo Noroeste, 4.- Canal Beagle.

No clear separation was noticed between CanalBrecknock, Canal Ballenero, Canal O’Brien andCanal Noroeste; waters were rather homogeneouswithin this sector but clearly distinct from CanalBeagle and open shelf Pacific waters.

A continuous gradient of increasing salinity fromPaso Ancho toward the eastern entrance of CanalBeagle is shown, when data of the entire study areaare depicted in a single profile along the inland pas-sages while ignoring connections to oceanic Pacificwaters. Nevertheless, some other oceanographicfeatures suggest the subdivision of micro basins. Forexample, Paso Ancho / Seno Magdalena had verti-cally mixed low salinity and cold water. BrazoNoroeste / Canal Ballenero had temperature inver-sions and two thermal and haline layers, with the

30 T. ANTEZANA

FIG. 6. – Vertical distribution of water properties along the transectCanal Beagle / Brazo Noroeste / Canal Ballenero / Canal Brec-knock / Pacific Ocean in spring 1994. a.- Density; b.- Salinity;

c.- Temperature.

warmest and saltiest waters at depth. Canal Mag-dalena / Canal Cockburn showed intermediate waterproperties with moderate salinity, temperature andvertical stratification. Canal Brecknock had clearlytransitional water conditions.

Finally surface temperature distribution in theupper 40 m throughout the entire region was ratherconstant around 6.5 ºC, which was about 1-3 ºClower than the Subantarctic surrounding shelfwaters. The glacier sector of Brazo Noroeste was theonly sector where a deep layer of warm waters sim-ilar to the surrounding shelf waters was detected.

Vertical stratification

Selected profiles of vertical distribution of tem-perature and salinity are depicted in order to stressdifferences between and to some extent withinmicrobasins. Maximum stratification occurred in theglacier sector e.g. off glaciers Francia and Garibaldi(Fig. 7a-b). Overlying a high salinity layer which

extended to the bottom, there was a strong haloclineabove 80 m, which reached the surface at glacierGaribaldi and broke into two shallow layers at glac-ier Francia. Temperature showed inversions associ-ated with several overlying layers: an upper warm(and brackish) layer less than 5 m thick over a coldand also brackish subsurface layer of 30-50 m, andboth over the warm deep layer of saline waterswhich extended to 250 m depth.

A more moderate stratification of salinity wasobserved in Canal Cockburn (Fig. 7c-d) where aweaker halocline was found at ca. 80 m; tempera-ture instead, was not stratified in this sector withthe exception of a slight gradient in the upper 30m. Homogeneously distributed waters appeared inSeno Magdalena and in Punta Yámana both withsimilar temperatures (ca. 6.6 ºC) but higher salin-ities in Punta Yámana than in Seno Magdalena.Moderate stratification or transition conditionswere found along Canal Brecknock and Canal Bal-lenero.

HYDROGRAPHY OF MAGELLAN AND FUEGIAN PASSAGES 31

FIG. 7. – Vertical distribution of temperature and salinity at selected sites of the Straits of Magellan - Beagle Chan-nel passage where the water column was strongly stratified next to the glaciers Ventisquero Francia and VentisqueroGaribaldi (a-b) and more or less homogeneous in Canal Cockburn, Seno Magdalena and Canal Beagle at

Punta Yámana (c-d).

DISCUSSION

Distribution of water properties was consistentwith general ocean current circulation indicating asouthward deflection of the West Wind Drift to formthe Cape Horn Current extending along the Chileanfjord region to Cape Horn. Although the entire lati-tudinal range was affected by the outflow of fresh-water from the inland passages, differences wereidentified in temperature, salinity and density distri-bution between the Pacific and Atlantic coastalwaters, as well as between the more northerly Straitsof Magellan and the southern Fuegian fjords on thePacific. One must bear in mind, however that thisstudy includes data from different cruises and doesnot consider seasonal variability. The lack of sam-pling through the connecting passages and in theimmediate open shelf waters in the Pacific is also alimitation.

Even so, limited water exchange between inlandpassages and the open ocean is suggested by thepresence of warm and salty waters in only some pas-sages and the direction of temperature, salinity anddensity gradients. Despite major connections to thePacific at the mouths of Canal Cockburn, CanalBrecknock, Canal Ballenero and Canal Beagle, thepresence of shallow sills here is likely to constrainwater exchange to the upper 30-40 m. In particular,the shallow depth of the eastern end of Canal Beagleapparently restricts the inflow of subsurface oceanwater. Since water exchange with the Pacific is lim-ited and is apparently taking place through narrowand shallow openings, most of the water filling theMagellan and Fuegian basins must be semi isolatedor trapped in this season.

Despite sampling depth restrictions (mainly inthe upper 200 m), profiles of water properties sug-gested that water exchange is less constrainedbetween inland passages than across the Pacificsills. Furthermore, other bathymetric features andlateral contractions within the inland passageswhich would affect water circulation, also explainthe hydrographic characteristics and subdivisions ofmicrobasins within this system. Exceptionally, thesill on the axis of Canal O’Brien which separatesBrazo Noroeste and Canal Ballenero had no appar-ent effect on water circulation. The use of bathymet-ric features to define microbasins in the Straits ofMagellan (Antezana et al., 1992) was consistentwith the distribution of water properties (Panella etal., 1991), copepods (Mazocchi and Ianora, 1991),microzooplankton (Fonda Umani and Monti, 1991)

and meso- and macrozooplankton (Guglielmo et al.,1991) in this passage.

One of the most obvious effects of bathymetry onhydrographic discontinuities took place at the con-fluence of Brazo Noroeste and Canal Beagle whereboth a depth and lateral contraction occur west ofPunta Yámana. Westward into Brazo Noroeste, thewater column was well stratified while eastward itwas homogeneous. Water characteristics of CanalBeagle at Punta Yámana (salinity 32.2 and tempera-ture 6.6º C through the water column) indicated aslight inflow of Pacific water, likely mixed with coldand fresh water from runoff and from the glaciers ofBrazo Noroeste. Eastward the influence of Pacificwaters was probably greater. Some preliminarystudies in early autumn suggest a westward inflowof even warmer and slightly saltier oceanic waterinto Canal Beagle (Celio, 1991). However the warmdeep layer (> 8.5 ºC) found in Brazo Noroeste /Canal Ballenero did not appear east of PuertoWilliams where depth is less than 50 m. Therefore,current data did not support the intrusion of Pacificwaters through Canal Beagle into Brazo Noroeste inspring.

The effect of melting water from several glaciersreaching the surface on the northern side of BrazoNoroeste (as evidenced by a vertically well stratifiedlayer of very cold and brackish water), was a signif-icant feature in the hydrography of these inland pas-sages. Vertical inversion was probably related towinter cooling and successive spring warming andalso to local cooling in the proximity of glaciers(Pickard, 1971, 1973). This cold and brackish layerdid not mix much with the overlying warm and saltywaters because of a strong pycnocline. Rather, thiscold and brackish layer slid down along Canal Bal-lenero to reach a depth of 100 m at the confluencewith Canal Brecknock. This glacier water mixedgradually with even less saline and warmer watersnear the Canal Brecknock / Canal Cockburn / CanalMagdalena system.

The influence of melting waters from Cordillerade Darwin on its northern and western sides was notevident in Canal Magdalena, (where Seno Keatsopens), but rather in the Seno Magdalena / PasoAncho sector where both Canal Gabriel and SenoAlmirantazgo open into this area. Therefore, thearea of confluence of the Straits of Magellan watersand the southern Magellan fjords system took placein Seno Magdalena where frontal conditions werefound (Punta Anxious / Isla Peak), during thisspring. The presence of a significant inflow of

32 T. ANTEZANA

brackish water was also suggested by Celio (1991)and Panella et al. (1991) at Canal Magdalena andSeno Magdalena during autumn and spring, respec-tively .

The underlying waters of Brazo Noroeste werethe warmest and saltiest found in the study area(8.05 ºC and 33.5 psu at the core). This body ofwater showed extreme values at the confluence withCanal Beagle (to the west of Punta Yámana) whichbecame an effective barrier for this apparentlytrapped water. Westward, this densest water (25.5.-26 kg/m3) clearly of Pacific origin spread throughCanal Ballenero and eventually may reach the CanalBrecknock / Canal Cockburn / Canal Magdalenabasin. These observations are consistent with anorthward inflow of Pacific waters suggested byCelio (1991). However the location of this inflow ofPacific waters is not consistent with shallow sills ofconnecting shelf passages. The Pacific water mighthave entered through Bahía Cook which is deeperand closer to the open Pacific than through otherconnecting passages, as a shallow dense water layersliding along the bottom of the basin. Alternatively,this water could have entered through the easternmouth of Canal Beagle in the summer, to cool downand descend in winter, to be finally trapped in theCanal Ballenero / Brazo Noroeste system.

Mixing of deep saline and overlying brackishwater took place along Canal Brecknock / CanalCockburn (deep salinity layer over 32 psu) andCanal Magdalena to reach homogeneous conditionsin Seno Magdalena / Paso Ancho. Various factorscould work together to produce such homogenousconditions. The combined effect of tidal currents,wind stress, internal friction and bottom friction toovercome the stratifying effect of freshwater dis-charge was suggested by Panella et al. (1991) in thewestern entrance of the Straits of Magellan. Indeed,tide and wind induced mixing may be equally effec-tive in wide basins such as Paso Ancho / Seno Mag-dalena and much less effective in narrow deepbasins such as Brazo Noroeste and Canal Ballenerowhere the stratifying effect of fresh water dischargemay be more pronounced.

In conclusion, patterns were identified in waterproperties which were associated with major bathy-metric features, including the presence of shallowconnecting sills to the Pacific ocean, which support-ed a subdivision of basins within the inland passagesystem, despite the assumed complexity of the studyarea, the short term of this study, and seasonal vari-ability. Profiles of water properties suggested that

water exchange is less constrained between inlandpassages than across the Pacific sills suggesting thatthis fjord system is rather semiclosed. These resultsmay be an important framework for understandingdistribution, linkages and biological adaptations oforganisms in the context of the multidisciplinaryeffort involved in the R.V. “Victor Hensen” cam-paign.

ACKNOWLEDGEMENTS

This research was funded by Alfred WegenerInstitute, the University of Concepción during theR.V. “Victor Hensen” cruise and JGOFS-Chile,SIDA-CONICYT project during succeeding stages.I am deeply indebted to co-workers Sergio Jara andYoanna Eissler who shared ideas, work and cheer-fulness on deck and in the laboratory. During the laststage of data processing and writing, I spent a sab-batical leave of absence at the Marine Life ResearchGroup of Scripps Institution of Oceanography andthe Coastal Fisheries Resources Division of South-West Fisheries Science Center, NOAA in La Jolla,where a wide source of support was provided. Spe-cial thanks go to Michael Mullin and John Hunter.My acknowledgements are also extended to RoyAllen for his dedication and skilful illustrations.Idiomatic improvement and editing of the manu-script was graciously done by long time friends andcolleagues Roger Leong and Kristen Ray to whom Iam always grateful, as well as to Ingo Wehrtmannand two anonymous referees.

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